Thermoelectric materials are those in which when used in certain combinations, can convert heat into an electric current. While thermoelectric devices can be used for either cooling (when an electric current is supplied) or power generation (when exposed to hot and cold temperature sources), the present invention is directed to the second case—power generation.
Thermoelectric materials have the potential to greatly increase the efficiency of power systems that rely on hydrocarbon fuels (for example, coal, gasoline, diesel fuel, etc.) as energy sources. While various power generating thermoelectric devices have been proposed over the years, they have not found widespread utility, particularly for mobile applications such as cars and trucks, due to their low efficiency and high cost. In power generation application, for example automotive applications, one encounters high temperature gradient situations. Currently, approximately 75% of the energy obtained from the combustions of a fuel such as gasoline is wasted due to thermal and other losses, and only about 25% of the energy is utilized by the vehicle for either moving it or powering equipment and accessories. Broken down, of the 100% of the energy from the fuel, approximately 40% is lost in the exhaust gases and 30% is lost cooling (radiator). Of the remaining 30%, approximately 5% is lost to friction or is radiated; leaving a net of 25% that is used for vehicle operation—mobility and powering (electrically) equipment and accessories. Of the 25% used for vehicle operation, about 18% is used for actually moving the vehicle and 7% is used for powering the equipment and accessories such as climate control, active breaking systems, entertainment (CD, radio, electronic engine control and other electrical power consuming equipment.
Presently, the electrical power demand of vehicles is increasing and is typically from 1-5 kilowatts in current vehicles. However, there is a problem due to the fact that the electrical power presently generated by a vehicle is generated very inefficiently, the efficient being less than 20% in the best instances, including chemical to mechanical and electrical conversion. A thermoelectric recovery system would enhance the efficiency of hydrocarbon-power vehicles by utilizing a part of the presently wasted thermal energy for direct electrical power conversion and would save the mechanical energy of the vehicles that is now used, for example by an alternator. This would result in a net saving of the overall chemical energy supplied by combustion of the fuel. Some of the benefits that would be obtained from the use of a thermoelectric module would include a reduction in environmental CO2, the ability to use lighter and less powerful engines due to decreased load on the engine to supply power, the use of smaller batteries because electrical power would be supplied from the thermoelectric module once the engine is started, and the ability to drop the need for certain equipment such as an alternator. The thermoelectric modules can be used in many different vehicular applications to increase the efficient of combustion engines, for example boats and airplanes as well as land vehicles, and they can also be used for non-mobile applications such as heating systems and emergency generators (small power units generally).
One of the problems with thermoelectric energy conversion is that it is not very efficient when there is a small thermal gradient, for example a gradient of 100-200 degrees Centigrade. The temperature range of from ambient to approximately 250° C. is the domain where most the presently existing thermoelectrical materials and devices operate, and they are generally only used in those situations where other, convention electrical generators are not preferred or cannot be used; for example, in satellites, pipelines, gas-line monitoring, and other others of similar nature where convention generators may present problems. In contrast, at higher temperature the thermoelectric efficiency rises in the Carnot cycle with the square of the thermal gradient (in degrees Kelvin, ° K).
The figure of merit (K−1) of thermoelectric materials is usually defined as:Z=S2σ/κ=S2//ρκ=PF/κwhere:                S is the Seebeck coefficient or thermopower (usually in μV/K)        σ is the electrical conductivity (in S.cm−1),        ρ is the electrical resistivity (in Ω.cm),        κ is the total thermal conductivity (in W.cm−1.K−1), and            PF is the electronic component or power factor (in W.K−2.m−1).
Since Z varies with temperature, thermoelectric materials (“TE”) materials are best rated by a dimensionless figure of merit, ZT, where T is temperature in K. Good TE materials have low thermal conductivity (e.g., phonon glasses), high electrical conductivity (e.g., electronic crystals), and high thermopower. However, these properties are difficult to optimize simultaneously in a specific material since the three parameters in Z are not independent. In general, as S increases, so does ρ.
The ideal maximum output power P of a module is given by the expression:P=(S·ΔT)2/(4ρL)where ΔT=Thot−Tcold is the difference of temperature between the hot and cold sources, and L is the length of the TE leg.
In conventional architecture, the choice of material for the electrode plate is led by electrical, thermal and thermo-mechanical considerations. That is, the material should be able to withstand high temperatures (resistance to creep and oxidation) and also achieve good heat transfer. The material should also be matched to electrode and TE elements, and be a good electrical insulator in order to prevent any circuit shortcut. Currently, alumina (Al2O3) and aluminum nitride (AlN) are generally favored as the plate materials. FIG. 1 illustrates a simplified drawing of a conventional thermoelectric module 10 utilizing materials such as alumina and aluminum nitride. QH and QC represent the heat input and output flows; Tsh and Tsc represent the hot source temperature and cold source temperature, respectively; ΔVoc is the voltage differential; α is a constant, and elements 12 are electric insulating layers.
The present invention is directed to solving the problem that in high temperature gradient applications, the present materials used for the electrode plate are exposed to severe thermal cycling (e.g. ranging from 50° to 850° C.) and are likely to create a prohibitive thermal expansion mismatch (see FIG. 2, right side). As a consequence of the thermal mismatch failures are likely to appear as shown in the right side of FIG. 2 which represents conventional thermoelectric module architecture 10. The left side of the dashed line represents the structure at room temperature, without any thermal gradient between the top and bottom of the Figure. The right side of the dashed line represents the same structure under an 800° C. thermal gradient when mismatched elements are used. For example, for a 50 mm large module, an 850° C. temperature gradient generates a 150 μm mismatch between the hot and cold alumina plate, presenting an 80×10−7/° C. CTE. Several complex architectures have been proposed to that fundamental problem as is disclosed by Tateyama et al., US 2005/0016183 A1 and US 2005/0211288 A1 (assigned to Kabushiki Kaisha Toshiba). The present invention offers a material solution preventing destructive high temperature mismatch between thermoelectric module hot and cold source.